Frictional resistance is generated at a hull of a sailing ship due to water. The frictional resistance is proportional to a submerged area of a hull submerged under water. A ship, especially a large ship, has a hull with a flat bottom surface. The bottom surface is a large portion of a surface area of the hull submerged under water.
An air layer may be formed at the bottom surface by air being discharged toward the bottom surface in order to reduce frictional resistance applied to the bottom surface. Specifically, air is supplied to a chamber installed on the bottom surface of the hull, and the air supplied to the chamber may be discharged toward the bottom surface through a plurality of air discharge ports provided at a lower portion of the chamber.